1. Field of the Invention
The present invention relates generally to forming a layer of synthetic corrosion products on tube surfaces, and more particularly to a method for forming a layer of synthetic corrosion products on tube surfaces using an object tube and a sacrificial tube with a different thermal expansion coefficient, the sacrificial tube having been previously shaped into any desired profile.
2. Description of the Prior Art
Corrosion products form during the operation of high temperature and pressure steam systems. These corrosion products accumulate on a steam system's heat exchanger tubing surfaces and affect the heat exchanger's efficiency and performance. In order to refine and improve the heat exchanger's operation, these deposits must be reproduced and tested under controlled conditions.
The corrosion products may be approximated by using a synthetic metal oxide mixture (sludge). This sludge may then be deposited onto a tube surface and the resulting composition used for developing and testing heat exchanger improvements. The tubes used in heat exchangers are generally very small in diameter and, currently, there is no viable method for depositing this sludge onto curved surfaces of such small diameter tubes.
This application deals mainly with adhering a layer of sludge products onto surfaces of small diameter tubes. Currently, there are two general methods for adhering a layer of material onto curved surfaces.
The first method involves introducing coating material into an annular cavity formed between an inner and outer mold. Pressure is then mechanically applied through expansion of one or both of these molds. These molds and expansion mechanisms are relatively bulky and are impractical for use with small diameter tubing used in heat exchangers.
The second method involves pre-forming coating material into tubes and inserting the tube into the object to be coated. The tube is then expanded, forcing the outer surface of the tube against an inner surface of the object to be coated with sufficient pressure to adhere the tube to the object's inner surface. This method is not applicable to heat exchanger tubing because sludge, by its very nature, is initially a slurry and cannot be pre-formed into an expandable insert.
The following U.S. Patents disclose the above methods for depositing a layer of material onto curved surfaces.
U.S. Pat. No. 2,771,655, by Nervi, discloses a method for manufacturing prestressed reinforced concrete piping. A cylindrical mold is placed within a bore of a pre-fabricated reinforced concrete pipe, leaving an annular cavity between an inner surface of the pipe and an outer surface of the mold. Fluid concrete is injected into this annular cavity followed by mechanical movement of the cylindrical mold to exert pressure on the fluid concrete. This pressure is transmitted through the fluid concrete to the pre-fabricated concrete pipe, causing the pipe to expand. This pressure is maintained until the fluid concrete has hardened. The pressure is then released and the pre-fabricated concrete pipe contracts to place the newly formed concrete lining in a state of compression. The resulting product is a pre-fabricated reinforced concrete pipe with a pre-stressed concrete lining.
U.S. Pat. No. 2,724,672, by Rubin, discloses a method for applying a thermoplastic polymer to the interior of pipes and tubes. The thermoplastic polymer is formed into a cylinder slightly smaller in outside diameter than an inside diameter of the pipe or tube. The polymer cylinder is sealed at both ends, with one end admitting a gas tube. Pressurized gas is introduced through this gas tube, causing the cylinder to expand against the pipe or tube with sufficient pressure to adhere the polymer tube's outer surface to the pipe or tube's inner surface. The pressurized gas is released and the ends of the cylinder are unsealed, leaving a pipe or tube with an inner layer of thermoplastic polymer.
U.S. Pat. No. 3,107,158, by Ahlberg, discloses a method for manufacturing pre-stressed concrete pipes. Fluid concrete is injected into an annular cavity formed between a flexible inner mold. and a rigid outer mold. Pressure on the fluid concrete is created by injecting water into the inner mold causing the mold to expand. The mold's expansion compresses the fluid concrete against the rigid outer mold and this pressure is maintained until the concrete has hardened. The molds are then removed and the result is a length of pre-stressed concrete pipe.
U.S. Pat. No. 3,382,121, by Sherlock, discloses a method for adhering an expansible polymer insert to an inner surface of a pipe or tube. The expansible insert is composed of a longitudinally oriented polymer which has been previously increased in length and decreased in width by extrusion, forming or cold drawing. This insert is placed within a bore of a pipe or tube and the assembly is heated to a temperature sufficient to release stresses induced by the polymer's orientation. The release of these stresses cause the expansible insert to retract in length and expand in width, thereby forcing the polymer's outer surface against the inner surface of the pipe or tube with sufficient pressure to bind the polymer to the pipe or tube. The result is a pipe or tube with an inner polymer lining.
U.S. Pat. No. 4,743,329, by Hata, discloses a method for manufacturing a plastic pipe with a thin lead lining. A lead pipe with an outer diameter slightly smaller than an inner diameter of a plastic pipe is placed within the bore of the plastic pipe. Both ends of the lead pipe are then sealed leaving only an opening for passage of pressurized gas. This gas is introduced into the lead pipe under sufficient pressure to expand the lead pipe until the lead pipe's outer surface comes into close contact, and adheres to, the plastic pipe's inner surface. The pressurized gas is released and the ends of the lead pipe are unsealed, leaving a plastic pipe with a lead lining.
U.S. Pat. No. 4,368,217, by Tournut, discloses a method for depositing a layer of polytetraflouroethylene (PTFE) onto an object's inner walls. A flexible membrane is placed within the object to be coated, leaving a space between an inner wall of the object and an outer surface of the flexible membrane. This space is filled with PTFE and compressed by deforming the flexible membrane under constant, i.e., isostatic, pressure. The membrane is removed and the object's internal cavity is filled with an incompressible solid substance having fluid flow characteristics and chemical stability at a temperature above PTFE's melting point. The assembly's temperature is raised above PTFE's melting point for a period of time sufficient for the PTFE to lose its crystallinity. The incompressible solid substance is removed leaving an object with an interior PTFE lining.